Photolithography or microlithography apparatus are widely used in the fabrication of microelectronic semiconductor devices and other microdevices. In photolithography, an optical system directs light energy to record a pattern at high resolution and with precise registration onto a photosensitive layer formed on a silicon wafer or other substrate. Continuing improvements in miniaturization place increasingly more challenging demands on the performance and accuracy of the optical system used for this function. Microlithography optical systems are fairly large and complex, containing a number of optical elements.
Lens assemblies used for microlithography, typically known as “stepper lenses”, typically comprise many lens elements, with each element accurately mounted in a cylindrical “cell” made of stainless steel or other stable material. Each of these cells can be precision-machined to extremely tight tolerances, with faces ground flat and parallel, for example, so that when the lens is assembled, each successive cell is fastened to the face of the adjacent cell. This type of arrangement generally allows only very limited adjustment, such as a small amount of tilt or centering motion in X and Y, for example. Once all the cells have been assembled, the entire lens is tested and any unwanted aberrations or image defects are identified. One stacked annuli lens assembly arrangement that is used for this type of optical apparatus is described, for example, in U.S. Pat. No. 5,428,482 entitled “Decoupled Mount for Optical Element and Stacked Annuli Assembly” to Bruning et al.
Final assembly of these lenses can be a time-consuming process, requiring iterations of testing and adjustment until proper performance levels are achieved. Some adjustments may require that a newly assembled lens be disassembled so that one or more components can be modified, then reassembled. Often, after such a complex lens assembly has been completely reassembled, it is then determined that one or more of the lens elements may need to be tilted or decentered slightly in order to correct other measurable optical defects. Some of the adjustments to correct for these defects would be best done with the lens assembled. It would be advantageous to provide a mounting mechanism that allows a degree of fine tuning of lens element positioning without requiring complete disassembly of system components. In designing a complex microlithography lens assembly, it is desirable to allow a measure of adjustability to specified lens elements so that particular defects can be corrected without introducing others. Specific defects or aberrations can be adjusted in a relatively “orthogonal” way if it is possible to make very fine tip and tilt adjustments to lens elements at one or more predetermined locations along the optical axis of the lens assembly, without requiring disassembly of the lens stack.
The task of compensating for aberrations in a complex microlithography assembly or other complex lens system can require significant time and resources and requires an initial detailed assessment of optical aberrations. Potential points of adjustment in the optical system can be identified based on an analysis of wavefront errors including, but not limited to, spherical aberration, coma, and astigmatism and of image placement errors, such as tangential; trapezoidal, and radial distortion and magnification error. As part of this assessment, adjustments that are likely to result in “crosstalk” effects between aberrations must be identified, so that in correcting one aberration, side effects that cause or increase other aberrations can be identified and minimized.
The sequence in which aberrations are compensated is highly significant. For example, in typical practice, after asymmetrical errors are first corrected to within reasonable tolerances, symmetrical errors are then compensated. Subsequent procedure then provides further correction for asymmetric errors. Repeated iterations may be necessary. To help minimize crosstalk between different aberration types, the orthogonality of different aberrations must be quantified in some way. By way of example, an assessment may determine that compensation by tilting a specific lens element may alleviate asymmetric coma and have very little effect on asymmetric astigmatism or tangential distortion. However, this same tilt may have a pronounced effect on trapezoidal distortion. Other adjustments in the system may have similar impact on other aberrations, where there is little effect in some cases or significant effects in others. For this reason, a systematic method is used for carrying out the sequence of adjustments that are needed for a specific lens assembly design. To reduce the likelihood of controlled correction without introducing errors, it is required that adjustments in this series be made without the need for disassembling and reassembling the lens.
There have been a number of solutions proposed for addressing the problems of tilt, tip, and various types of decentering adjustments. Some of these solutions require complex mechanical assemblies having many small parts, such as that shown in U.S. Pat. No. 5,986,827 entitled “Precision Tip-Tilt-Piston Actuator that Provides Exact Constraint” to Hale. Other solutions, such as that proposed in U.S. Pat. No. 6,271,976 entitled “Apparatus for Tilting an Object About at Least One Axis, In Particular an Optical Element” to Weber, provide arrangements of constraints that kinematically define two orthogonal axes of rotation along with adjustable constraints for setting the tilt angle or tilt and tip angles.
Although such conventional approaches can achieve some measure of control over tilt and tip adjustment, however, they fall short of what is needed for high-resolution microlithography. Complex mechanical assemblies can be costly to manufacture and use, can be subject to thermal expansion effects, and can introduce unwanted motion “impurities” that limit their precision and accuracy. Constraint arrangements must be carefully designed and assembled in order to prevent overconstraint in any direction. There is thus a need for an improved optical element mount that allows fine tuning of lens element tilt and tip position and decentration after assembly without the requirement for taking the lens assembly apart.